1. Field of the Invention
The present invention relates to a switching power supply of a synchronous rectification type and an image forming apparatus on which the power supply is mounted.
2. Description of the Related Art
There has been a demand to improve efficiency of an operation of a power supply in recent years in order to save power in electronic appliances. As one example of the power supply that realizes improvement of operation efficiency, a power supply is discussed in Japanese Patent Application Laid-Open No. 7-115766. Illustrated in FIG. 8 is one example of configuration of the conventional power supply. Referring to FIG. 8, the power supply includes a direct current power supply 1, a control circuit 2 controlling a metal oxide semiconductor field effect transistor (MOS-FET, hereinafter also referred to as FET) serving as a switching element, a field effect transistor (FET) 4 as a main switching element, a transformer 5, a FET 6 which is a second switching element, a smoothing capacitor 7, a load 8 to which a voltage is supplied, an output voltage detection circuit 9, a body diode 10 parasitic in the FET 6, a driving circuit 11 for the FET 6, and a current detection circuit 12 for the FET 6.
An input voltage from the direct current power supply 1 is accumulated as an excitation energy in a primary coil n1 of the transformer 5 by conducting (turning on) the FET 4 controlled by the control circuit 2 and the driving circuit 3. Meanwhile, the energy excited by the primary coil n1 of the transformer 5 when non-conducting (turning off) the FET 4 is converted to the energy in a secondary coil n2 to be supplied to the load via the body diode 10 and the current detection circuit 12. The current detection circuit 12 sends an on-signal to the driving circuit 11 upon detection of a flow of the current to turn on the FET 6. Further, the current detection circuit 12 determines that the current has finished flowing when the current flowing therein becomes lower than a predetermined value, and sends an off-signal to the driving circuit 12 to turn off the FET 6. A loss caused by the FET 6 is remarkably lower than a loss by a forward voltage of the body diode 10. Therefore, improvement in efficiency is attained by performing control in such a manner that the current to be supplied to the body diode 10 is switched to the FET 6. This method is called synchronous rectification, and the FET 6 is also called synchronous rectification FET.
However, the following issues have been found in the above-described conventional example. In the case of the above-described synchronous rectification, the circuit for detecting on/off of the synchronous rectification FET is provided in the configuration for the purpose of improving efficiency of the operation, and the detection circuit is complicated. Therefore, a circuit size is increased, which causes a cost increase. Also, in the case of detecting the current flowing in the secondary coil by the current transformer and performing the on/off control of the FET by the detected current, an amount of the current flowing in the secondary coil is small at the time of a light load (light load state), so that the current output from the current transformer is reduced to the very small current, thereby making it impossible to correctly control the on/off of the FET. When the on/off of the FET is not correctly controlled (driven), efficiency may be reduced or power consumption may be increased. The light load state means a standby state of awaiting an operation or a power saving mode (also called sleep mode) for reducing power consumption, which is a so-called default state in electric appliances. Since the default state is the state that occupies a considerable time in a day in an electronic device, the increase in power consumption and the loss caused by the on/off operation of the FET greatly influence a total power consumption of the electronic appliance.